Kaolinite based pigments are commonly used in paper industries for paper filing and paper coating applications. In general, the objectives of using the pigment are to improve appear qualities, such as opacity, brightness, smoothness, printing, porosity, surface coverage, light scatter, and to reduce the coast of paper manufacturing.
Both the brightness characteristics of the given kaolin and the opacifying properties of same when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S." The said parameter, i.e., the scattering coefficient S of a given filler pigment, is a property well-known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539 (1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature. Reference may usefully be had in this connection e.g., to U.S. Pat. Nos. 4,026,726 and 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology Vol. 2 "Paper," Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, N.Y.).
In a filled paper, higher light scattering is therefore important. Increased light scatter allows paper to look more opaque without increasing light absorption. The use of pigment with a higher light scattering coefficient allows reduction in either the basis weight or amount of filler required to achieve targeted properties, for example, opacity and brightness. Traditionally, this has been achieved using titanium dioxide, calcined clays and precipitated calcium carbonate. The relatively higher light scattering of titanium dioxide is due to higher refractive index. Higher light scatter observed with calcined kaolin and precipitated calcium carbonate is believed to be due to the intrinsic porous structure developed during the process of manufacturing of these pigments. See McConnell et al, U.S. Pat. No. 4,381,948.
In general, the attempt to increase light scatter by modification of kaolinite mineral also induces some increase in pore void volume. In addition, such modification can produce pigments with particle size distribution in a fairly narrow range. For example, calcining of fine kaolinite above its dehydroxylation point can produce a product with increased pore void volume. In U.S. Ser. No. 918,632 filed Oct. 14, 1986, similar aggregation is achieved chemically by reacted fine kaolinite clay with rapidly hydrolyzing metal chlorides. The acidic by-product of this reaction may be neutralized with gaseous ammonia. The light scattering coefficient and pore void volumes of these clays are significantly higher than the starting kaolinite material. Marginal increase in light scatter (generally less than 10 units) may be induced by mixing kaolinite particles of different size or by chemical flocculation. However, these structures are generally unstable and would break down under high shear stress of paper making or paper coating.
Aside from use as fillers, the aggregated pigments are used in paper coating to improve surface coverage. The application of such pigments can lead to a smoother surface, higher porosity, gloss and print properties. In the said patent application a chemically aggregated kaolin pigment is shown to significantly increase coated sheet properties, especially paper and print gloss.
In more detail, in U.S. Pat. No. 4,381,948 to A. D. McConnell et al, a calcined kaolin pigment is disclosed and a method for manufacture of same. The said pigment consists of porous aggregates of kaolin platelets, and exhibits exceptionally high light scattering characteristics when incorporated as a filler in paper. This pigment, which substantially corresponds to the commercially available product ALPHATEX.RTM. of the present assignee, E.C.C. America Inc. (Atlanta, Georgia), is prepared by first blunging and dispersing an appropriate crude kaolin to form an aqueous dispersion of same. The blunged and dispersed aqueous slurry is subjected to a particle size separation from which there is recovered a slurry of the clay, which includes a very fine particle size; e.g. substantially all particles can be smaller than 1 micrometer E.S.D. The slurry is dried to produce a relatively moisture-free clay, which is then thoroughly pulverized to break up agglomerates. This material is then used as a feed to a calciner; such feed is calcined under carefully controlled conditions to typical temperatures of at least 900.degree. C. The resulting product is cooled and pulverized to provide a pigment of the porous high light scattering aggregates of kaolin platelets as described.
Calcined kaolin products, including those of the aforementioned ALPHATEX.RTM. type, are seen to be manufactured by relatively complex techniques involving a multiplicity of steps, including specifically a calcining step, plus various preparatory steps and post-calcining steps. Thus, the said product is relatively expensive to produce; and requires considerable investment in complex apparatus and the like--e.g. highly regulated calciners, etc. It can indeed be noted that the conditions of preparation of these materials must be very carefully controlled in order to keep abrasion acceptably low in the calcined product. For example, the calcination operations tends per se to produce an abrasive product--in consequence of overheating--if great care is not taken to preclude such a result.
It is further to be noted that in order to produce a low abrasion calcined product, the particle size in the feed to the calciner must be carefully controlled--even a relatively small increase in coarseness of such feed can have very marked detrimental effect on Valley abrasion.
In U.S. Ser. No. 918,632 filed Oct. 14, 1986, a process is disclosed in which a fine particle size kaolin is reacted in particulate form with a metal chloride, such as silicon tetrachloride, to form a chemically aggregated structured kaolin pigment. The metal chloride may be one or more of the chlorides having the general formula MCl.sub.x, where M is Si, Ti or Al; and X is 3 or 4 depending on the valence of M. Heating may optionally be used to shorten the reaction time. When so used, temperatures generally will not, however, exceed about 150.degree. C. In order to complete the polymerization and condensation which is though to occur, it is preferable to age the resulting product for a period, typically at least three days. In another aspect of that process, additional improvements in the products are found to occur by the addition of ammonia to the combined kaolin and metal chloride.
Thus said patent application describes methods of preparing chemically aggregated kaolinite mineral using very reactive metal chlorides such as silicon tetrachloride and titanium tetrachloride. The application of such reactive metal chloride leads to aggregated products that give enhanced optical and printability properties for both filled and coated papers. However, due to the nature of the aggregating chemical, very specialized equipment and processing steps are required. In addition, the resultant by-products are difficult to remove by ordinary methods. In this invention, similar aggregates may be produced by reaction with organo-silicon compounds that are non-corrosive and produce no solid by-products.
It is known from U.S. Pat. No. 3,567,680 to Joseph Iannicelli, assigned to J. M. Huber Corporation, issued March 2, 1971, that mercaptopropyl silanes having the formula: ##STR1## wherein Z is selected from the group consisting of hydrogen, cation, alkyl, aryl, alkylaryl, arylalkyl and derivatives thereof; X is selected from the group consisting of alkyl, alkylaryl, and arylalkyl; and R.sub.1, R.sub.2 and R.sub.3 are selected from the group consisting of hydrogen, cation and alkyl, are suitable for modifying kaolin clays to enable them to be used as reinforcing fillers for elastomers. In fact the thus modified clays have been the candidates of choice for such fillers in commerce. It may be noted that in the Iannicelli disclosure, only the trialkoxy mercaptopropyl silanes are considered. Blends of these mercapto organosilanes with amino organosilanes are also disclosed.
In U.S. Pat. No. 3,364,059 to Marzocchi, a method for treating glass fibers to improve their bonding relationship to rubbers comprises treating them with a silane containing a thio group.
According to the present invention, the thio group and the amino group are not required. Sulfur-free and nitrogen-free organic silicon compounds are employed.
In U.S. Pat. No. 3,834,924 to Thomas G. Grillo, assigned to J. M. Huber Corporation, an amino organosilane is added to a high solids content pigment dispersion or slurry to change the slurry form into a thick, flocculated and plastic type that is suitable for extrusion and drying. Because a thick, cake-like product is formed, the amino organosilane and pigment dispersion are preferably mixed or blended directly in a solids mixing apparatus such as an extruder, designed to extrude the plastic mass in the form of a compacted rod type body which may be fed directly into a drier. The products are useful as a filler for polyurethanes. As can be seen, the described treatment is for the different purpose of forming a flocculated, plastic mass of the kaolin, not for the purpose of aggregating fine kaolin particles to form aggregated fine kaolin particles.
In U.S. Pat. No. 3,894,882 to Robert B. Takewell et al, assigned to J. M. Huber Corporation, a rotating pelletizing drum is used to form pellets from clay such as kaolin clay. To avoid the problem of dust, a wetting liquid is introduced into the drum, preferably steam or steam/water. The steam adds heat to the pellets to aid in drying them. There is an incidental mention of using "other suitable wetting liquids", an extensive list being given which includes silanes.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide an aggregated kaolin pigment product which possesses improved pigment bulk, porosity and light scattering characteristics, and hence is useful as a bulking pigment for coating of paper and paper board, and which may also be used as an opacifier and light scattering filler for paper and paper board as well as in other paper manufacturing applications.
It is also an object of the invention to provide a pigment product of the foregoing character which is prepared without calcination and therefore without subjecting the kaolinite to high temperatures, and which accordingly possesses low abrasiveness in accordance with the kaolinite feed from which it is produced.
Another object of the present method is to demonstrate application of substantially dry kaolin pigment in aggregation.
Yet another object is to demonstrate aggregation of kaolinite particle by dry processing routes.
Yet another object is to demonstrate use of aggregated pigment made by the present invention in paper filling to enhance optical properties.
Yet another object is to show the application of such pigment in light weight coatings.
Yet another object is to show the production of the pigment in the presence of an aggregation enhancer such as calcium chloride.
It is a further object of the present invention to produce pigment that is free of soluble salts.
It is yet another object of the present invention to produce a pigment that, as a consequence of aggregation, can be processed to a slurry at a solids content higher than 60% by weight without unduly poor rheological consequences.
It is yet another object of the present invention to provide a process wherein aggregation efficiency, i.e. light scatter, is improved by application of gaseous ammonia.
It is an object of the present invention to produce aggregated kaolinite pigment with minimum steps in producing said pigment.
It is also an object of the present invention to produce aggregated kaolinite pigment at relatively low cost compared with available such products.